Tailoring material composition for optimization of application-specific switchable holograms

ABSTRACT

The present invention offers control over—and the ability to optimize—the performance parameters of switchable holograms. The present invention offers the ability to tailor the properties of a hologram to a desired application and thus make application-specific holograms. The invention relates to polymer-dispersed liquid crystal materials subject to control and optimization of the performance parameters of switchable holograms. Such variability allows tailoring the properties to application-specific devices. Specifically, the present invention provides an improved polymer-dispersed liquid crystal system that allows variation of: 1) haze, 2) switching voltage, 3) electrical power dissipation, 4) switching stability (voltage creep), 5) switching contrast ratio (range), 6) dynamic stability, and 7) the operating temperature range.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application incorporates by reference in their entirety thefollowing United States Patent Applications similarly titled “SWITCHABLEVOLUME HOLOGRAM MATERIALS AND DEVICES”: Ser. No. 09/033,512 filed Mar.2, 1998, now U.S. Pat. No. 6,699,407; Ser. No. 09/033,513 filed Mar. 2,1998; Ser. No. 09/033,514 filed Mar. 2, 1998, now U.S. Pat. No.6,667,086; Ser. No. 09/034,014 filed Oct. 29, 1999, now U.S. Pat. No.6,706,086; Ser. No. 09/429,645 filed Mar. 21, 1998, now U.S. Pat. No.6,667,134; and Ser. No. 09/347,624 filed Jul. 2, 1999, now U.S. Pat. No.6,692,666. Also incorporated herein by reference in their entireties areU.S. patent application Ser. No. 09/742,397 filed on Dec. 22, 2000entitled “SWITCHABLE POLYMER-DISPERSED LIQUID CRYSTAL OPTICAL ELEMENTS,”and Ser. No. 09/577,166 filed on May 24, 2000 now U.S. Pat. No.6,730,442, entitled “A SYSTEM AND METHOD FOR REPLICATING HOLOGRAM.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to photopolymerizable materials,and more specifically to polymer-dispersed liquid crystal materials inwhich the switchable hologram performance parameters are subject tocontrol and optimization.

2. Description of the Related Art

Demand for information has become a strong driver in many business,consumer, and government applications. Three key components of thisdemand are the storage, transmission, and display of information. Thelatter two in particular are placing severe demands on availablehardware and software. In communications, there has been an explosion oftraffic driven by the Internet, business data, and digital imagetransfers. In the end-point use of this huge data stream, visualutilization and management of data have high priority. Large datacontent requires high resolution (super video graphics array (“SVGA”) toextended graphics array (“XGA”)) along with full-color capability. Thetechnological response to these challenges has spawned severalinnovations. For telecom applications, part of the response is toprovide higher data rates and bandwidth extension through the use ofdense wavelength division multiplexing (DWDM). For easy visual access toinformation, portable and handheld devices are evolving along with flatscreens and personal displays. In addition, efforts are underway to makethe advantages of digital video disc (“DVD”) and high definitiontelevision (“HDTV”) available in these formats.

Optics is at the core of all of these technologies. The informationrevolution is placing stringent demands on several optical components.For example, short- and long-period fiber Bragg gratings are playing keyroles in the telecom industry, but the demand for multiple wavelengthsand the ability for dynamic reconfiguration by DWDM is growing. Ininformation display applications, the use of portable andmicro-displays, combined with virtual display technology, is creatingthe need for complex off-axis optical systems in very compact,lightweight packages. This becomes impossibly heavy and cumbersome withconventional refractive and reflective optics.

Diffractive optics is the natural response to many of these demands. Butthese devices are by their very nature monochromatic. Multi-wavelengthand dynamic reconfiguration capabilities are forcing a reconsiderationof the use and fabrication of diffractive optical elements to satisfythe growing needs of the information revolution.

This revolution is creating demands for efficiency across a wide-rangeof applications. An approach developed to achieve such efficiency isthrough application specific performance, focusing on the parametersimportant for individual applications. Among these applications are:fiber optic switches; reprogrammable N×N optical interconnects foroptical computing; beam steering for laser surgery; beam steering forlaser radar; holographic image storage and retrieval; digital zoomoptics (switchable holographic lenses); graphic arts and entertainment;and the like.

Switchable holographic optical elements (HOEs) have been invented tofulfill the promise of diffractive optics in meeting the technologicalchallenges in telecom and information display. Multi-layered switchableholographic optical elements in a single solid-state device form asubstitute for multiple static elements and complexrefractive/reflective optical systems.

A hologram is an interference pattern that is recorded on ahigh-resolution recording plate. Two beams formed by a coherent beamfrom a laser interfere within the recording plate, causing aninterference pattern. This pattern represents object information. Theobject information is a function of the light diffracted from the objectto be recorded when the object is placed in the path of one of the twoformation beams. If the resulting recording plate is viewed correctly inmonochromatic light, a three-dimensional image of the object—ahologram—is seen. When forming a holographic grating, there is noobject, per se, which is put into the path of one of the beams. Instead,given the wave properties of light, when two beams interact, they willform a grating within the recording plate. This grating can be formed soas to have any of a variety of characteristics.

SUMMARY OF THE INVENTION

Summary of the Problem

Switchable hologram technology must present a flexible approach tooptical element design and fabrication, offering high efficiency andoptical quality with low power consumption. Moreover, it must betailored to customer specifications, i.e., it has to be veryapplication-specific. For example, devices in telecom applications thatrequire specific wavelength and format consideration includereconfigurable add/drop switches, multiplexers, optical cross connects,optical switches, wavelength selectors and tuners, and spectralattenuators or gain flatteners. Examples of such needs also abound inthe information display area, including personal DVD/HDTV viewers,portable displays, data phone/handheld Internet displays, wearable PCdisplays, digital picture frames, desktop telephone E-mail/Internetdisplays, ultra-portable projection systems, and desktop monitors.

As the switchable holographic materials and devices discussed hereinnear application in the markets discussed herein, it has become clearthat several performance parameters are critical. In particular,parameters require control and independent optimization for devices tomeet the demanding requirements of anticipated applications in thetelecom and display industries. It is just such control and optimizationthat will tailor properties of switchable HOEs for theapplication-specific needs of customers. Most of these parameters relateto intrinsic properties of materials and/or processes. Theseconsiderations go beyond obvious modifications of materials and devices.For example, it is well known in the art that the thickness of aswitchable hologram can be modified to either increase diffractionefficiency or reduce switching voltage. There is, of course, a trade-offbecause one case involves increasing the thickness while the otherrequires a decrease in thickness.

In many applications with holograms, haze is a problem. Inpolymer-dispersed liquid crystal (“PDLC”) holograms haze is produced bylight scattering from inhomogeneities in the film. Some of these arecontaminants that can be controlled by careful processing. Othersoriginate from phase-separated LC droplets. The diffraction planesthemselves will produce some random scattering due to nonuniformdistributions of LC droplets from plane to plane. But a major source ofscattering comes from phase-separated droplets that occur outside thedesired Bragg planes. Examples of this are cross-gratings anddiffraction rings formed by spurious reflections and diffraction of therecording beams. Also, in some cases, LC may randomly phase separate inthe polymer-rich regions. Scattering is a strong function of dropletsize and density. A haze as large as 10% has been measured. It isstrongly desired to reduce and control the amount of haze in hologramsfor specific applications.

In electrically switchable holograms, power dissipation is anotherparameter of interest. When high, it can lead to joule heating, which insome cases can cause problems with thermal stability. Large powerconsumption requires a more expensive electrical power supply. It canalso require larger voltages, which may lead to electrical shorting thatdestroys the usefulness of the hologram. This depends largely on theswitching voltage—high switching voltage leads to large current drawnfrom the power supply. In switchable PDLC gratings, power consumptionand dissipation comes from current drawn to charge up the transparentelectrodes, and resistive heating in the transparent electrodes andthrough the hologram (due to a finite conductivity of the PDLCmaterial).

Thermal stability of holographic PDLC (“H-PDLC”) material is also aproblem. Above the liquid crystal nematic-to-isotropic (N-I) transitiontemperature there is reduced diffraction efficiency for the H-PDLC, butthe index mismatch cannot be removed by application of an electricfield. Upon reducing the temperature below the N-I transition the DE isrestored to its maximum value and the grating can be switched clear byapplying a field. Factors such as usage environment, source temperature,or optimal operating temperatures for certain applications have led to aneed for better understanding of the H-PDLC performance at elevatedtemperature. Furthermore, some applications require that they performcontinuously at elevated temperatures whereas others require that theH-PDLCs do not fall below certain minimum specifications over anextended range of temperatures. Commercially available LC mixtures aretypically eutectic solutions that are formulated to provide a broadnematic temperature range. However, some LCs perform better than othersas the typical operating temperature is raised. For example, when astack of reflection gratings is subjected to an intense broadband sourceof light, the source produces a significant heat load such that, evenafter extensively filtering the source to reduce infrared radiation, thetemperature of the filters is well above ambient. It is strongly desiredto develop H-PDLC material with good and/or variable thermal stability.

Tanaka et al. (K. Tanaka et al., U.S. Pat. No. 5,751,452 and 5,748,272)teach an optical device made from a switchable holographic PDLC gratingand methods for fabricating the same. They provide little detail aboutmaterials or methods other than standard holographic methods well knownby those practicing the art. They do teach use of NOA65 (polyene andpolythiol mixture), but not how it may be used in conjunction with amultifunctional acrylate to reduce switching voltage and significantlyreduce voltage creep. NOA65 is the only polymerizable monomer in theTanaka et al. system; Tanaka et al. provides nothing new over prior art(see Margerum et al., U.S. Pat. No. 4,938,568 and #5,096,282).

Crawford et al. (G. P. Crawford et al., U.S. Pat. No. 5,875,012, andEuropean Patent Applications #98300541.1, #98300543.0, #98300468.0)teach reflective displays made with switchable PDLC holograms, butprovide little in the way of materials or methods for optimizingperformance. They teach the use of an anisotropic polymer index-matchedto the liquid crystal (LC) to reduce haze at large viewing angles.Crawford et al. provide nothing new over prior art in PDLCs (see Doaneet al., U.S. Pat. Nos. 4,994,204 and 5,240,636).

Taketomi et al. (Taketomi et al., U.S. Pat. No. 5,731,853) and Ninomiyaet al. (Ninomiya et al., U.S. Pat. No. 6,083,575) teach devices madewith switchable PDLC holograms, but provide no teaching for optimizingswitchable hologram performance.

These considerations are well known, but the prior art has not addressedspecific intrinsic material compositions or processing controlparameters, or other external controls, that can systematically optimizethese parameters to meet the specific requirements of variousapplications. In fact, independent and optimal control of all of theseparameters is just what is required to tailor the material and deviceproperties to application-specific situations. How this may be achievedhas not been described in the art. The embodiments of the presentinvention described herein detail compositions and methods for precisecontrol and optimization of parameters specific to individualapplications.

Summary of the Solution

PDLC holographic materials described herein offer a solution to the needfor an electronically driven, multi-layer, multi-wavelength, complexoptical system in a thin, lightweight, low-electrical-power element.

Embodiments of the present invention provide material formulations forimproving the performance of polymer-dispersed liquid crystal (PDLC)holograms in application-specific situations.

According to particular embodiments of the present invention, a secondmonomer, in proper relative concentration with the multifunctionalprimary monomer, combined with a second phase separation is used toachieve improved switching and voltage creep performance.

According to further embodiments of the present invention, haze isaddressed without the need for relying on anisotropic properties of thehost polymer.

Particular embodiments of the current invention also usemulti-functional acrylates which form a three-dimensional network withelastic properties resulting in desired LC properties. The LC can besqueezed into a separate phase yielding desirable optical properties forthe hologram. The multi-functionality also leads to continualpost-polymerization after the hologram recording is completed whichstiffens the matrix and increases the switching voltage (i.e., voltagecreep). The elastic relaxation of the multi-functional acrylate systemalso produces another phenomenon: shrinkage. Shrinkage has a majoreffect in a direction parallel to the grating vector and thus—inreflection gratings—this reduces the grating period. Controllingshrinkage in the plane of the hologram can prevent “wrinkling” of thehologram and therefore precludes certain defects in the surface of thehologram.

Further embodiments of the present invention include wavelengthselective optical elements formed by processes and materials that aretailored so as to adjust and achieve desired performance parameters. Theadjustable parameters include haze, voltage, electrical powerdissipation, switching stability, switching contrast ration,environmental stability and operating temperature range.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will be more clearly understood from a reading ofthe following description in conjunction with the accompanying figureswherein:

FIGS. 1A and 1B show a switching field comparison for a reflectiongrating formulated without a secondary monomer (FIG. 1A) and with asecondary monomer (FIG. 1B) according to an embodiment of the presentinvention;

FIG. 2 shows response and relaxation time measurements for a sampleaccording to an embodiment of the present invention; and

FIG. 3 shows the switching voltage as a function of time followingholographic recording within a sample according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE PRESENTINVENTION

As switchable holographic materials and devices near application in themarkets discussed above, it has become clear that several performanceparameters are critical. Specifically, parameters listed in Table 1require control and independent optimization for devices to meet thedemanding requirements of anticipated applications in the telecom anddisplay industries. Such control and optimization of switchable HOEsproperties will result in products able to meet the application-specificneeds of customers. The embodiments described herein describe theprecise control and optimization of parameters specific to individualapplications. Such tailoring is achieved via variation of the qualitieslisted in Table 1 through a combination of material compositions andcontrol of molecular diffusion, phase separation, morphology, and otherprocessing aspects during the fabrication of switchable holograms.

TABLE 1 Performance Parameters Materials/Process Control Haze LC dropletsize Switching voltage (electric field) Matrix conductivity LC/polymerinterfacial elasticity Electrical power dissipation Matrix conductivitySwitching stability (voltage creep) LC/polymer interfacial elasticitySwitching contrast ratio (dynamic Matrix index range) LC ordinary indexEnvironmental stability High N-I temperature LC Operating temperaturerange Wide temperature range LC Diluent concentration

A basic component of the optical elements described within thisdisclosure is the PDLC material used therein. A general description ofthe ingredients which comprise this PDLC material—and various specificexamples of combinations of these ingredients used to form specifictypes of PDLC materials are discussed below. The PDLC material generallycomprises a monomer, a dispersed liquid crystal, a cross-linkingmonomer, a coinitiator and a photoinitiator dye. These PDLC materialsexhibit clear and orderly separation of the liquid crystal and curedpolymer, whereby the PDLC material advantageously provides high qualityholographic gratings. The PDLC materials of the present invention arealso advantageously formed in a single step. The present invention alsoutilizes a unique photopolymerizable prepolymer material that permits insitu control over characteristics of the resulting gratings, such asdomain size, shape, density, ordering, and the like. Also, methods andmaterials of the present invention can be used to prepare PDLC materialsthat function as switchable transmission or reflection gratings.

PDLC materials, methods, and devices contemplated for use in thepractice of the present invention are also described in R. L. Sutherlandet al., “Bragg Gratings in an Acrylate Polymer Consisting of PeriodicPolymer-Dispersed Liquid-Crystal Planes,” Chemistry of Materials, No. 5,pp. 1533–1538 (1993); in R. L. Sutherland et al., “Electricallyswitchable volume gratings in polymer-dispersed liquid crystals,”Applied Physics Letters, Vol. 64, No. 9, pp. 1074–1076 (1984); and T. J.Bunning et al., “The Morphology and Performance of HolographicTransmission Gratings Recorded in Polymer-dispersed Liquid Crystals,”Polymer, Vol. 36, No. 14, pp. 2699–2708 (1995), all of which are fullyincorporated by reference into this Detailed Description. Alsoincorporated by reference herein are U.S. Pat. Nos. 5,942,157 and5,698,343.

The process by which a hologram is formed according to the invention iscontrolled primarily by the choice of components used to prepare thehomogeneous starting mixture (hereafter “syrup”), and by the intensityof the incident light pattern. A preferred PDLC material employed in thepractice of the present invention creates a switchable hologram in asingle step. The resulting preferred PDLC material has an anisotropicspatial distribution of phase-separated LC droplets within thephotochemically cured polymer matrix. The features of the PDLC materialare also influenced by the components used in the preparation of thehomogeneous starting mixture and by the intensity of the incident lightpattern. In a preferred embodiment, the prepolymer material comprises amixture of a photopolymerizable monomer, a second phase material, aphotoinitiator dye, a coinitiator, a chain extender (or cross-linker),and, optionally, a surfactant. Highly functionalized monomers are alsopreferred because the extensive cross-linking associated with suchmonomers yields fast kinetics. Highly functionalized monomers, however,are relatively viscous. As a result, these monomers do not tend to mixwell with other materials, and they are difficult to spread into thinfilms. One preferred embodiment of the present invention uses a mixtureof acrylates. Suitable acrylates, such as triethyleneglycol diacrylate,trimethylolpropane triacrylate, pentaerythritol triacrylate,pentaerythritol tetraacrylate, pentaerythritol pentaacrylate, and thelike can be used in accordance with the present invention. The secondphase material of choice for use in the practice of the presentinvention is a LC. This also allows an electro-optical response for theresulting hologram. Preferred PDLC materials are discussed in detail inUnited States Patent Applications entitled SWITCHABLE POLYMER-DISPERSEDLIQUID CRYSTAL OPTICAL ELEMENTS and A SYSTEM AND METHOD FOR REPLICATINGVOLUME HOLOGRAMS incorporated by reference above.

The material comprising this invention is a mixture of a polymerizablemonomer and liquid crystal, along with other ingredients, including aphotoinitiator dye. Upon irradiation, the liquid crystal separates as adistinct phase of nanometer-size droplets aligned in periodic channelsforming the hologram. The material is called a holographicpolymer-dispersed liquid crystal (“H-PDLC”). Both transmission andreflection type holograms may be formed with H-PDLCs. One skilled in theart recognizes that the same concepts embodied in this disclosure areviable for other applications, including those for communicationsswitches, switchable transmission and reflection lenses (red, green, andblue). Preferred embodiments of the present invention use a PDLCmaterial and its unique switching characteristics to form opticalelements. The PDLC material of the present invention offers all of thefeatures of holographic photopolymers plus the hologram can be switchedon and off with the application of an electric field. Most importantly,additional advantages include control over the following parameters: 1)haze, 2) switching voltage, 3) electrical power dissipation, 4)switching stability (i.e., voltage creep), 5) switching contrast ratio(i.e., dynamic range), 6) dynamic stability, and 7) the operatingtemperature range.

Some aspects of the present invention include a homogeneous mixture ofingredients (i.e., syrup) given generically by the following: one ormore polymerizable monomers (mixture of multi-functional acrylates,including at least a pentaacrylate), a LC (typically a mixture ofcyanobiphenyls), a photoinitiator dye (one dye with absorption spectrumoverlapping recording laser wavelength), a co-initiator, a reactivediluent or cross-linking agent, and, optionally, a surfactant orsurfactant-like additive. When this system is irradiatedholographically, the photoinitiator absorbs light in the bright fringesand reacts with the co-initiator, creating free radicals. The freeradicals then initiate polymerization of the multi-functional acrylates.The free-radical process is very fast, and a three-dimensional polymernetwork is created in just a few seconds. This rapid development of adensely cross-linked network is critical to the phase separation of LCdroplets in the dark fringes, which is what establishes the hologram.Highly functional acrylates are needed to produce this with a minimum ofexposure time. The surfactant contributes to matrix conductivity and thereduction of LC/polymer anchoring, thereby reducing the switchingvoltage. However, it also acts as a diluent to the LC, reducing itsorder parameter and lowering the nematic-isotropic transitiontemperature. This can dramatically affect the temperature stability ofthe hologram.

In an embodiment of the present invention, multi-functional acrylatesare used. They form a three-dimensional network with elastic propertieswhich can cause “squeezing” of the LC—this squeezing causes the LC tomove into a separate phase yielding desirable optical properties for thehologram. The strong elastic forces involved make the polymer matrixvery stiff which adds to a high switching voltage for the H-PDLChologram. The multi-functionality leads to continual post-polymerizationin the dark after the hologram recording is completed which furtherstiffens the matrix and causes “voltage creep”—a slow increase in theswitching voltage. In some cases it can increase the switching voltageby as much as 100%.

In many applications with holograms, haze is a problem. The presentinvention improves haze produced by light scattering frominhomogeneities in the PDLC film. Some embodiments of the presentinvention combat scattering associated with phase-separated dropletsthat occur outside the desired Bragg planes. Scattering is a strongfunction of droplet size and density. Switching speed requirements willdepend on the application. Some applications may requireon/off-switching times in the microsecond regime, while some may onlyrequire millisecond response. It is useful to be able to tailor theswitching speed to the application to optimize other parameters, such asswitching voltage. Embodiments of the present invention retain desirableattributes of the multi-functional acrylate system for forming PDLCholograms, but add new materials to the syrup or new process controls torecording to optimize performance parameters as may be needed forspecific applications. This results in high optical quality switchableholograms with good diffraction efficiency and low, stable switchingvoltage.

In one embodiment of the present invention, the hologram material allowscontrol over the amount of scattering via control of the LC dropletsize. Droplet size a impacts scattering loss; at any wavelength, thescattering coefficient increases with size approximately as a³. There isalso a trade-off of switching voltage with relaxation time. (SeeEquations (1) and (2)). It is desirable to keep a as small as possible.Since scattering and relaxation time are approximately proportional toa³ and a², respectively, while switching voltage is proportional to a⁻¹,much may be gained by minimizing a. But this will begin to increaseswitching voltage unfavorably (even when optimizing matrix conductivityand interfacial anchoring see below). At some point it is desirable tooffset decreases in a with some other parameter.

In this embodiment, reducing LC droplet size is achieved by lowering theinterfacial tension between the LC and the surrounding polymer. Reducingthe interfacial tension lowers the amount of work required to producedroplets of high surface area-volume ratio when the LC drops out of thepolymer network as a separate phase. This can be controlled with theaddition of an interfacial tension agent to the starting syrup. Agentsincorporating a hydroxyl group are desired because they may readily formhydrogen bonds with side chains of the acrylate polymer. The other endof the molecule should mix well with the LC. Hydrocarbon chains performthis well because LCs also have long hydrocarbon chains. Linear and/orbranched acids of the type C_(n)H_((2n+1))COOH with n≦18 are effectiveas interfacial tension agents. With interfacial tension agentconcentrations up to about 10 wt-%, LC droplets decrease in diametermonotonically and droplets in samples containing no interfacial agentwere ˜100–150 nm in diameter. However, samples containing 4–6 wt-% ofinterfacial agent had smaller droplet diameters (˜20–40 nm dia.). Higherconcentrations have produced droplet diameters as small as 10 nm.Usually, as the agent concentration is increased, droplet size decreasesdrastically and is not easily detected using scanning electronmicroscopy. Switching voltages also increase dramatically, indicatingvery small droplet diameters (i.e., <<10 nm). Much research has focusedon analyzing the optimal concentrations of various interfacial tensionagents on both optical quality and switching properties. Concentrationsin the 2–7 wt-% range are useful for a variety of organic acids, but theprinciple of this invention is not necessarily limited to the use ofacids. Other polar molecules that are preferentially attracted to theoxygen in the acrylate side chain and have long hydrocarbon chainscompatible with the LC also function as effective interfacial tensionagents for controlling LC droplet size.

In a further embodiment of the present invention, switching voltage isreduced through the addition of an additive monomer acrylate, e.g.,branched alkyl chains, to the H-PDLC material. Branched alkyl chainscontaining acrylates such as t-butyl acrylate, t-butyl methacrylate,isopropyl acrylate, and isopropyl methacrylate, lower the surfaceanchoring interactions with the liquid crystal due to the steric effectimposed by the bulky alkyl groups close to the vinyl double bond. As aresult, the LC droplets are less strongly bound to the polymer surfaceenabling the reorientation of the LC droplet when an electric field isapplied, thus reducing the amount of required switching voltage.

In another embodiment of the present invention, the H-PDLC materialallows control over the switching voltage via control of theconductivity of the matrix. The switching and voltage creep phenomenaare best discussed in the context of a simple model. The switchingvoltage of a switchable hologram is related to the critical electricfield (E_(c)) necessary to reorient the LCs. This critical field can beestimated by the following equation: $\begin{matrix}{E_{c} = {\frac{1}{3_{a}}{\left( {\frac{\sigma_{LC}}{\sigma_{p}} + 2} \right)\left\lbrack \frac{\overset{\_}{k}\left( {l^{2} - 1} \right)}{\Delta ɛ} \right\rbrack}^{\frac{1}{2}}}} & (1)\end{matrix}$

Equation (1) predicts the critical field for an elongated LC droplet,with semi-major axis a, semi-minor axis b, and aspect ratio l=a/b.σ_(LC) and σ_(p) are the electrical conductivities of the LC andpolymer, respectively; {overscore (k)} is an average elastic forceconstant while Δε is the dielectric anisotropy, both considered constantproperties of the bulk LC. This equation can identify properties totarget for reducing the switching voltage, though certain constraintswill apply. An arbitrary change in one parameter can adversely affectother parameters so care must be taken in modifications to the syrup.The modifications described below either retain performance parametersat their optimum level or enhance them. Also, the droplet size a must bekept small to keep scattering losses down and the aspect ratio l canalso be controlled but may be traded off against other parameters (e.g.,polarization dependence or index modulation). Other parameters to targetinclude the conductivity ratio, the force constant, and the dielectricanisotropy.

Cyanobiphenyls are known to contain ionic impurities that contribute tothe conductivity of the LC. These are difficult to eliminate. However,the conductivity of the polymer matrix may be modified by increasingσ_(p) which lowers the conductivity ratio and reduces the critical fieldfor switching. Addition of the surfactant (e.g., octanoic acid) hasalready reduced this ratio some by increasing the matrix conductivitybut surfactants also tend to dilute the LC and lower its orderparameter, thereby reducing the nematic-isotropic temperature.Therefore, this invention introduces a monomer that can be chemicallyincorporated into the acrylate network and it increases the conductivityof the polymer matrix. One example of this type of monomer is acrylicacid. Acrylic acid is a weak organic acid (pK_(a)=4.25) that increasesthe conductivity of the polymer matrix by contributing a free proton tothe matrix. This process is described by the following equilibriumreaction:RCOH

RCO⁻+H⁺where R is an acrylate incorporated by a covalent bond into the polymernetwork.

The conductivity of the matrix may increase from incorporation of thistype of monomer such that the critical field—and thus the switchingvoltage of the film—is significantly reduced. Illustrative examples aregiven in the table below.

TABLE 2 Switching Switching wt-%-Acrylic Voltage Current, I_(rms) Samplewt-% Surfactant Acid (V_(rms)/μm) (mA) 1 4.5 0 14.1* 6 2 4.5 1 12.6* 6 34.5 1.5 6.9 5.9 4 6 1 9.07 6.8 *Based upon 8 μm nominal thickness

Incorporation of such monomers is therefore useful in reducing theswitching field necessary for clearing H-PDLC devices. Results basedupon analysis of the grating devices (i.e., the holographically recordedfilm contained between transparent electrode-coated substrates) indicatethat an optimum concentration can be reached whereby the relative ratiosof surfactant and conducting monomer have optimal switching voltage andpower consumption as described below.

When power dissipation in electrically switchable holograms is high, itcan lead to joule heating, which in some cases can cause problems withthermal stability. Large power consumption also requires a moreexpensive electrical power supply and larger voltages—which may lead toelectrical shorting that destroys the usefulness of the hologram. Inswitchable PDLC gratings, power consumption and dissipation comes fromcurrent drawn to charge up the transparent electrodes, and resistiveheating in the transparent electrodes and in the hologram (due to afinite conductivity of the PDLC material). Power consumption is relatedto the electrical power density P dissipated as heat in the hologram. Atthe critical field, this is given approximately by P=σ_(p)E_(c) ². Asthe matrix conductivity increases, σ_(p) goes up linearly while E_(c) ²goes down quadratically. Therefore, for minimum power consumption, thereis an optimum value σ_(p) given by setting the derivative dP/dσ_(p)=0.This yields the optimal relation σ_(p)=σ_(LC)/2. The effect of theconductivity ratio may be estimated by using Equation (1) to calculatethe critical field in the idealized situation when the conductivityratio is zero, and comparing that to the actual situation shown bysample 1 in Table 2. Assuming typical LC droplet parameters learnedthrough electron microscopy of samples (a˜0.06 μm, b˜0.04 μm, l˜1.5),and values of parameters for the LC BL045 (see above) used in the syrup[{overscore (k)}=(k₁₁+k₃₃)/2=16.75×10⁻¹²N, Δε=15.3ε₀, ε₀=8.85×10⁻¹²F/m],the estimation is E_(c)˜4 V/μm (vs. an experimental value of 14 V/μm,though experimental values have ranged from 14–18 V/μm). This impliesthat the conductivity ratio in the samples without the acrylic acidmonomer is ˜5–7. Addition of 1.5 wt-% high conductivity monomer reducedthe switching voltage by a factor of 2, which implies that theconductivity ratio decreased correspondingly to ˜1.5–2.5. This is closeto the optimum ratio given above for minimum power consumption. Furtherreduction can be achieved by reducing E_(c) but holding σ_(p) constant.

In other embodiments, monomers other than weak acids as described in theexample may be employed which increase conductivity upon incorporationinto the polymer matrix. Examples of materials that may be of useinclude reactive ionic materials, aromatic materials which provideelectron delocalization, or monomers which are used to make conductingpolymers such as conjugated aromatics and polypyrroles. Also, the use oflong and intermediate chain-length acrylated or methacrylated acids ofthe type (H₂CCH—(CO)O—R—(CO)OH, where R is a linear or branched alkylchain may be effective. These materials may provide similar improvementsin matrix conductivity to those already described. Again, an aspect ofthis invention is the control of matrix conductivity to lower switchingvoltage without contaminating or diluting the LC droplet.

In yet another embodiment of the present invention, the hologrammaterial allows control of the switching voltage and voltage creep viacontrol over the LC/polymer interfacial elasticity. When bulk values ofthe elastic force constant are substituted into Equation (1), anassumption that the interface has no effect on the elastic properties ofthe droplet is required. This is equivalent to the case of stronganchoring in ordinary LC cells—the surface interaction energy of the LCand polymer is infinite. This may be similar to the case in thepentaacrylate system discussed above, because (a) the interactionbetween the LC and the polymer may be mediated by hydrogen bonds, whichare generally stronger than van der Waals forces, and (b) thepentaacrylate polymer is very stiff. This shows that the surface energycan be lowered by establishing an interface at the phase-separated LCthat would form weaker bonds and be more elastic. A potential problemwith this approach is that favorable properties of the free-radicalinitiated multi-functional acrylate system, which gives such gooddiffraction grating properties, may be surrendered. This invention usesthe following approach to solve this problem.

A second monomer, which polymerizes by a different chemistry than thatof the multi-functional acrylate is added to the syrup in relatively lowconcentration. In particular, its polymerization is dominated by a slow,step-growth process, and may be initiated by an external stimulus thatis different than the radiation used to record the hologram.Alternatively, it may be weakly initiated with the recording radiation,but its rate of polymerization is slow compared to the rates ofdiffusion and phase separation. These monomers and/or oligomers arechemically compatible with the LC, because they remain in solution withthe LC and diffuse and phase separate with the LC when the hologram isrecorded. After the hologram is recorded, the second polymerizationprocess is initiated, or caused to accelerate, by the application of anexternal stimulus. A second phase separation may occur where the secondmonomer, because of its lower surface energy, preferentially polymerizesabout the surface of the LC droplet, forming a sheath that separates theLC from the multi-functional acrylate polymer. This polymer (a) does notreadily form hydrogen bonds with the LC molecules (it interacts with LCmolecules through the relatively weaker van der Waals forces instead),and/or (b) is a soft, more elastic interface that “gives” more in itsinteraction with the LC (i.e., it is less stiff than themulti-functional acrylate polymer).

The effect of this second polymer sheath is two-fold. First, since itlowers the interfacial energy, it also reduces the effective elasticforce constant of the LC. As shown by Equation (1), it also reduces thecritical field for switching (i.e., it lowers the switching voltage).Second, it buffers the effects of the multi-functional acrylate polymeron the elastic properties of the LC. In other words, aspost-polymerization proceeds in the acrylate polymer—stiffening thenetwork with time—the interface between the LC and the second polymer isnot affected, it remains soft and more elastic. Since the interfacialelastic properties remain constant with time, there is significantlyless voltage creep.

A trade-off is required to obtain this benefit. A stiffer medium, thoughmore difficult to deform by an applied field, snaps back to its originalform more rapidly because of the strong elastic restoring force—itsrelaxation time is very short. The same elongated droplet model leadingto Equation (1) predicts a relaxation time (when the applied field isturned off) given by $\begin{matrix}{\tau_{off} = \frac{\gamma_{l}a^{2}}{\overset{\_}{k}\left( {l^{2} - 1} \right)}} & (2)\end{matrix}$where γ₁ is the rotational viscosity coefficient of the LC. Thus, areduction in the effective elastic force constant that produces areduction in the critical field by a factor of M will tend to increasethe relaxation time by a factor of M². If the longer relaxation time isstill compatible with the switching time needed for a particularapplication, then the slower relaxation is not a severe penalty.

The second type of monomer described in this invention may polymerize ina secondary process following recording of the hologram. In a preferredembodiment of the present invention, this monomer polymerizes via athiol-ene step-growth mechanism upon ultra-violet (UV) illumination ofthe film (i.e., these materials contain a UV photoinitiator). Inalternative embodiments of the present invention, other polymerizationmechanisms and/or other initiation methods such as visible light, heat,electron beams, or the presence of reactants could be used to achievethe intended effects. In a preferred embodiment, concentration rangesare 30–50% primary monomer, 30–50% liquid crystal, 0.1–2% initiator andco-initiator, 5–15% reactive diluent, 0–10% surfactant-like material,and 5–20% added/secondary monomer. In other embodiments, the secondarymonomer can be a commercially available monomer such as a Norlandoptical adhesive (“NOA”) from Norland Products, for example, NOA 61, NOA76, NOA 65, NOA 83H, NOA 88 and the like.

Another effect has been on the switching characteristics. In theexamples given in FIG. 1, for an otherwise identical pre-polymerformulation, the switching voltage of a sample that does not contain thesecondary monomer, i.e., thiol-ene, was approximately 15 V/μm, while theswitching voltage of a sample formulated with the secondary monomer,i.e., thiol-ene, was 6 V/μm after UV post-cure—a reduction in switchingvoltage of 60% was observed. This indicates a 40% decrease in switchingvoltage (almost a factor of 2) over the best results observed in theprior art.

As discussed above, the primary cause of the observed voltage reductioncan involve reduction of the LC anchoring with the polymer “sheath”produced by the added secondary monomer. The decreased anchoring canreduce the effective elastic force constant ({overscore (k)}) containedin Equations (1) and (2). This reduced anchoring, as described, can alsoto lead to an increase in the relaxation time (τ_(off)). FIG. 2 is anexample of the response and relaxation time measurements for a samplethat does not contain the secondary monomer, i.e., thiol-ene. In thisspecific embodiment, the response times for the sample not containingthe thiol-ene material and for the sample containing it are similar at48 μs and 52 μs, respectively. For this same specific embodiment, therelaxation times are markedly different however, with values of 223 μsand 707 μs for the non-thiol-ene and thiol-ene containing films,respectively.

The required switching field at the time of recording is an importantfactor in determining the overall usefulness of a sample, though theswitching field after a normal use period may be more determinative.Further to the specific embodiment, switching voltage recorded at thetime of recording increases by as much as 100% after 7 days of storagein the dark. Using the method described above to reduce switchingvoltage, this voltage creep has decreased significantly. Representativeexamples are shown in FIG. 3—the post-recording voltage in theformulation described increased by approximately 27%. In syrupscontaining the thiol-ene monomers the voltage increase was greater than47% when the sample was not post-cured with UV. But identical sampleswith thiol-ene monomers showed no significant increase when they were UVpost-cured for several minutes, which shows that the increase inswitching voltage is arrested by the addition of these monomers to thesyrup and subsequent post-cure.

Further, the incorporation of secondary monomers which cure in asecondary process allow for the use of previously unacceptable LCmaterials with varying optical or chemical properties for optimizingH-PDLCs for specific applications. For example, LC material TL213 whenincorporated without a secondary monomer, requires an extremely highswitching voltage. Where as the same LC material, when incorporated intoa test mixture that also includes a secondary monomer, such as one ofthe NOA monomers, led to improved phase separation, and higherdiffraction efficiencies. Certain LC materials, such as TL213, have a Δnvery similar to other LC materials, such as BL045 or other BL series LCmaterials, but the Δε of TL213 is significantly lower. Gratings madewith this LC material, or other materials having similar Δn and Δεcharacteristics, switch at prohibitively high voltages when used in thesingle monomer embodiments described above. This prohibitively highswitching voltage rendered components using these materials, without theaddition of a secondary monomer, unusable for many applications. On thecontrary, and in accordance with this embodiment of the presentinvention, upon incorporating a secondary monomer, such as thosedescribed above, into the PDLC syrup, the switching voltage is loweredand the H-PDLC components maintain the improved characteristics stemmingfrom the TL213 LC material. One skilled in the art recognizes thecurrently available equivalent LC materials which may be used on boththe single monomer and the double monomer embodiments described herein.

Again, the secondary monomers are used to preferentially segregate withthe LC during holographic recording. Within the LC droplets thesecondary monomers act as diluents, reducing the order parameter anddisrupting the phase behavior. Upon UV illumination, the growingsecondary monomer polymers phase separate from the LCs and coat thesurfaces of the droplets in a thin layer. The interaction between thesecondary polymer and the LC is less than that observed for the primarypolymer with the LCs. This causes the reduced switching voltage of theH-PDLC. Furthermore, the secondary polymer coating on the droplets canshield the LC from the effects of the primary polymerpost-polymerization, thereby reducing the voltage creep. Also, the phaseseparation of the secondary polymer from the LC eliminates or reducesits diluent effect, which causes the increase in diffraction efficiencysometimes observed upon UV illumination. One skilled in the artrecognizes that multiple combinations of primary and secondary monomersand initiation methods are contemplated by the scope of this invention.

Thus, features of the embodiments discussed above include theincorporation of a step-growth monomer into the free-radical monomersystem that polymerizes using a different chemistry than thefree-radical host system, a step-growth monomer that preferentiallydiffuses with (and remains miscible with) the LC during holographicrecording—and curing of the multi-functional free-radical polymer—thatcan be later polymerized by a second initiation mechanism which isfollowed by a second phase separation where the step-growth polymerforms an elastic sheath about the LC droplet such that it reduces theeffective elastic constant of the LC —thereby lowering the switchingvoltage—and buffers the LC from the effects of post-polymerization inthe multi-functional polymer host, thereby reducing voltage creep.

In yet another embodiment of the present invention, the hologrammaterial allows control of the contrast ratio and/or dynamic range ofthe material via control over the matrix index and LC index. Manyapplications require very high contrast between the intensity of thelight diffracted from a grating in the field-off versus the field-onstate. To increase the contrast ratio it is necessary to match n_(LC)closely to n_(p) at a particular applied voltage. In an embodiment ofthe present invention, alkyl chain molecules are added to the syrup tomatch the indices of the LC droplets and the polymer surrounding thePDLCs in the voltage on-state. Alkyl chain molecules are chemicallycompatible with LC molecules, which also have long alkyl chains. Thealkyl chain additives preferentially phase separate with the LC andthereby modify the effective refractive index of the droplets such thatin the voltage on-state the droplet index is substantially matched tothe surrounding polymer index, producing a minimum diffractionefficiency.

The refractive index of the LC droplet will vary between maximum(n_(max)) and minimum (n_(min)) values as an electric field is applied.To obtain zero diffraction efficiency, n_(min) should be exactly matchedto the surrounding polymer index n_(p). In samples comprised of thesyrup discussed above, n_(min)>n_(p). Compounds having smaller indexn_(a) mixed with the LC will lower the effective LC index and achieve(n_(mix))_(min)≈n_(p). Using the following standard Clausius-Mosattirelation for the refractive index of a mixture of dielectric materials,n=n ₂ +f(n ₂ −n ₁)  (3)(n_(mix))_(min)˜n_(p) can be approximated as follows:(n _(mix))_(min) ˜n _(p) ˜n _(m)+((f _(a)/(f _(a) +f _(LC))) (n _(a) −n_(min))  (4)wherein f_(a/)f_(a)+f_(LC)=f_(a·LC). And the requiredrelative-volume-fraction f_(a:LC) of additive (a) to liquid crystal (LC)can be estimated as follows for the refractive index of a mixture ofdielectric materials, when the difference in indices of the twomaterials is ≦0.5: $\begin{matrix}{f_{a \cdot {LC}} = {\frac{n_{\min} - n_{p}}{n_{\min} - n_{a}}.}} & (5)\end{matrix}$Note that n_(min) and n_(p) must be obtained from measurements, (i.e.,n_(min) usually from diffraction efficiency measurements and n_(p)usually from refractometer measurements).

Other embodiments of the present invention use long chain alkylcompounds as potential surfactants for the mixture. In some cases, asurfactant itself can serve as the additive for index matching. Inothers, another alkyl molecule not comprised of a polar head can serveas the additive. It should be noted that a surfactant has specificproperties for controlling the interfacial anchoring of the LC-polymerinterface, but when used as an additive, its intended function is tomodify the refractive index of the LC droplet. The surfactant moleculecan serve dual purposes in some cases, but it is not absolutelynecessary that the additive be a surfactant.

Although the photoinitiator and co-initiator start the polymerization,they are minor components in the mixture. The major components, otherthan the polymerizable monomer, are the LC, the cross linker, the longchain alkyl additive and the surfactant (if added). All of thecomponents are originally mixed homogeneously. When the mixture issubjected to the light of an interferogram, photopolymerization ispreferentially started in the bright fringes of the interferogram.Monomers are thus consumed to form new species as polymer and oligomerchains start to grow. The cross linker participates in this chemicalreaction and is thus also preferentially consumed in the bright fringeregions. Hence, the concentrations of monomer and cross linker in thebright regions decrease relative to their respective concentrations inthe dark regions of the interferogram. By Fick's law, this gradient inconcentration of species causes a diffusion of monomer and cross linkerfrom the dark regions to the bright regions where they in turn alsoparticipate in the chemical reaction. At the same time, constituentswhich are not consumed in the chemical reaction (e.g., LC and additivemolecules) increase their relative concentrations in the bright regionsrelative to the dark regions because a number of monomer and crosslinker molecules have been consumed in the chemical reaction while thenumber of LC and additive molecules within the same spatial domain haveremained unchanged, at least temporarily. Fick's law applies again,driving the diffusion of LC and additive molecules away from the brightregions and into the dark regions of the interferogram. The monomer andcross linker diffuse to the bright regions, and the LC and additivediffuse to the dark regions.

As the cross link density increases, the molecular weight of the polymergrows, and the polymer network spreads out into the dark regions of theinterferogram. Eventually, the mixture of LC with growing polymerbecomes unstable, and the LC separates out as a distinct phase. This canoccur as nucleation of small LC droplets with subsequent growth bydiffusion of LC molecules. Growth is terminated when the polymer networkbecomes sufficiently rigid. However, because of the anisotropicdiffusion of LC into the dark fringe regions, the LC phase-separatesthere into distinct channels, forming a volume hologram.

The ratio of additive to phase-separated LC can remain the same afterphoto-curing as it starts out in the uncured mixture. In this situation,the overall volume fraction of additive f_(a) needed in the mixture canbe estimated in terms of the starting LC volume fraction f_(LC) and therelative volume fraction f_(a·Lc): $\begin{matrix}{f_{a} \approx {\frac{f_{LC}f_{aLC}}{1 - f_{a.{LC}}}.}} & (6)\end{matrix}$

This generally gives a lower bound on the amount of additive requiredsince one cannot guarantee that the additive and LC will stay in thesame relative ratio as they phase separate. An alternative is obtainedby measuring the actual index difference Δn=(n_(mix))_(min)−n_(p) formore than one value of the starting ratio f_(a)/f_(LC). A plot of thesedata is formed, and a curve is fit to the data. The desired startingvalue of f_(a)/f_(LC) is found as the curve of Δn vs. f_(a)/f_(LC) isextrapolated to cross the Δn=0 axis.

By way of example, a group of low index, long alkyl chain molecules withtheir respective refractive indices n_(D) at the sodium D-line is givenin Table 3 below. The compounds in the bottom part of the table, asindicated by the dotted line, are also surfactants. The compounds in thetop of Table 3 are simple long chain alkyls with increasing chain lengthdown the table. In all cases, the refractive index increases withincreasing chain length. In general, the lower the refractive index, theless additive required to match LC and polymer indices.

TABLE 3 Additive n_(D) Nonane 1.405 Decane 1.411 Dodecane 1.422 Undecane1.417 Tridecane 1.425 Tetradecane 1.429 Pentadecane 1.434 Hexadecane1.434 Heptadecane 1.436 Hexanoic acid 1.4161 Heptanoic acid 1.4224Propyl pentanoic acid 1.4250 Octanoic acid 1.4278

In one embodiment of the present invention, this technique uses octanoicacid as the additive for index matching (not as surfactant in thisembodiment). The measured refractive index difference at switching forf_(a)=0.0 and 0.04 (f_(LC)˜0.3), when combined with equations (5) and(6), leads to the prediction that f_(a)˜0.0635 yielding a ˜0%diffraction efficiency. Actual volume fractions and minimum diffractionefficiency (n_(min)) measurements are shown in Table 4 below.Measurements were performed at a wavelength of 633 nm. With minorchanges of the additive and more careful control over parameters, it ispossible to achieve n_(min)<0.01% (i.e., <0.0001).

TABLE 4 Volume Fraction of Octanoic Acid η_(min) 0.0 0.05 0.04 0.0060.0634 0.00017

Adding diluents to the LC can have a negative impact on the thermalproperties of the grating due to a lowering of the LC order parameter.According to this embodiment of the present invention, small amounts ofco-monomers are added to the syrup to vary the refractive index of thepolymer matrix. The refractive index of a copolymer can be estimated byknowing the index of each of the homopolymers and the weight fraction ofthe monomers and a more explicit solution can be obtained using theLorentz-Lorenz equation:(n ²−1)/(n ²+2)=ρr  (7)

where n is the refractive index, ρ is the density, and r is the specificrefractivity, each of the individual homopolymer. Given the refractiveindices n_(A) and n_(B) of the homopolymers A and B, and theircorresponding densities PA and PB, their specific refractivities may becalculated from Eq. (7). The specific refractivity of the co-polymer(two polymers A and B) is found from r_(AB)=f_(A)r_(A)+(1−f _(A))r_(B),where f_(A) is the weight fraction of polymer A. Using this value ofspecific refractivity in Eq. (7), along with the density of theco-polymer ρ_(AB)=f_(A)ρ_(A)+(1−f_(A))ρ_(B), the co-polymer refractiveindex n_(AB) may be predicted. An analysis such as this allows one toselect a polymer of index n_(A) and weight fraction f_(A) to modify therefractive index of the main polymer from n_(B) to n_(AB).

Monomers for use in this embodiment include fluorinated mono-acrylatesand methacrylates (e.g., hexafluoroisopropyl acrylate (n ˜1.42)).Fluorinated mono-acrylates have very low refractive indices with respectto the high functionality aliphatic monomers that are currently used.Other monomers include the n-alkyl mono-acrylates such as octyl acrylateand decyl acrylate. As with the fluoro-acrylates, these long chainmono-acrylates have refractive indices lower than that of the primarypolymer and therefore reduce the overall refractive index of the H-PDLCmatrix. In contrast, materials such as aromatic acrylates and thosemonomers containing the heavier halogens (e.g., bromine and/or iodine)have higher refractive indices than that of the primary monomer and cantherefore be used to increase the overall refractive index of the H-PDLCmatrix. Some examples of this type of monomer includetribromo-phenylacrylate and pentabromo-phenylacrylate with refractiveindices of approximately 1.6 and 1.7 respectively.

In the embodiments mentioned above, chemically compatible materials(e.g., long chain alkyls to the LC, or acrylates to the polymer) tocontrol the index match between n_(min) of the LC droplet and n_(p) ofthe surrounding polymer—such that the additive materials have minimalimpact on the other properties of the switchable hologram—may be added.

Some applications may place the switchable PDLC hologram in harshenvironments that may degrade its properties. Typical environmentalparameters that prove deleterious to operation include temperature,humidity, and UV exposure, although the most severe of these is probablytemperature. LCs nominally have freezing points below 0° C. andnematic-to-isotropic (N−1) transition points at 65–100° C. The hightemperature range is usually the most problematic in devices. Anycontaminants or diluents in the LC will lower its order parameter andthereby reduce its N−1 transition. This in turn can significantly reducediffraction efficiency. For example, the N-I transition may be reducedby as much as 30–40° C. by such contaminants/diluents. This severelyrestricts the operating temperature of the hologram.

In yet another embodiment of the present invention, the hologrammaterial allows control over thermal stability via control of thediluent and after processing of the H-PDLC. Commercially availableliquid crystal mixtures are typically eutectic solutions that areformulated to provide a broad nematic temperature range. However, aswould be expected, some LCs perform better as the typical operatingtemperature is raised. This embodiment includes two LCs. The first LC (amixture of cyanobiphenyls) has acceptable diffraction efficiency butlower switching voltage at room temperature. However, afterincorporating the reflection-grating stack into the system, thisformulation underwent a LC phase transition below the operatingtemperature of the holograms in the device. The second LC (a mixture offluorinated biphenyls) has a higher diffraction efficiency and higherswitching voltage, but it also possesses a higher N−1 transition. Uponincorporating H-PDLCs made with the alternative liquid crystal into thefilter stack and operating at the elevated temperature, it has a wideroperating temperature and better diffraction efficiency at elevatedtemperature, but the elevated temperature also reduced the viscosity ofthe LC leading to lower switching voltages which were more favorable forthe application. The incorporation of additives into the syrup candepress the transition temperature of the LC, narrowing the nematicrange. Also, LCs with a wider nematic range can give betterelectro-optical properties throughout an array of operatingtemperatures, even if the properties at ambient temperatures (e.g.,switching voltage) are not as favorable. Finally, for applicationsinvolving operation at a given temperature, the choice of liquid crystaland other formulation components can be optimized for that specifictemperature.

This embodiment includes the addition of surfactants—which act asdiluents and lower the order parameter of the LC—which lower the N-Itransition of the H-PDLC. In some embodiments, the surfactants are longalkyl chain acids. In two preferred embodiments, decanoic (C₁₀ chain)and dodecanoic (C₁₂ chain) acids are used. Using the syrup previouslydiscussed but substituting decanoic acid or dodecanoic acid for theoctanoic acid increased this temperature range to ˜50° C. Withdodecanoic acid the concentration can be lowered to 4-wt % and theswitching voltage can be reduced by a factor of ˜2 at elevatedtemperatures while the diffraction efficiency decreases by only ˜3%.

The embodiments discussed herein also increase the temperature rangediscussed above. In a preferred embodiment, a combination of white lightexposure (several hours in room lights) and heat cycling (25–75-25° C.)extends the temperature range of the diffraction efficiency withswitching voltage reducing at operating temperature. An example of thisembodiment is shown in Table 5 for syrups containing 4-wt % and 3-wt %dodecanoic acid (DDA). The samples were exposed to room lights for 240hours and temperature-cycled as described above twice. In thisembodiment, the maximum operating temperature (i.e., the point where thediffraction efficiency decreases by only 3% from room temperature) isextended to 55–60° C. while the switching voltage has decreased by afactor greater than 2. Before treatment, the samples had a maximumoperating temperature of 39° C.

According to an embodiment of this invention, the operating temperatureof an H-PDLC grating is increased while keeping the switching voltage ata low and reasonable value. This was accomplished by: 1) substituting along chain acid (e.g., dodecanoic acid) for the common surfactant(octanoic acid), 2) heat cycling and white light exposure to extend theoperating temperature of the H-PDLC grating (i.e., extending thetemperature range over which the grating has diffraction efficiency lossof ≦3% compared to its room temperature value), and 3) taking advantageof the decrease in LC viscosity and/or elastic force constants withtemperature to reduce the switching voltage at operating temperature toa low, reasonable value.

TABLE 5 3% DDA Sample 4% DDA Sample Temperature DE Sw. VoltageTemperature DE Sw. Voltage 25 40.8 140 25 43.4 174 27 40.7 138 27 43.2174 29 40.6 132 29 43.4 165 31 41.0 129 31 43.6 159 33 40.7 126 33 43.7160 35 40.5 126 35 43.9 157 37 40.7 126 37 43.9 156 39 40.7 115 39 43.9146 41 40.6 115 41 43.8 138 43 40.7 108 43 43.8 129 45 40.7 105 45 43.5120 47 40.6 96 47 43.3 117 49 40.4 93 49 42.7 111 51 40.0 90 51 41.7 8553 39.9 81 53 41.2 84 55 39.7 78 55 40.2 81 57 39.1 70 57 37.0 70 5937.9 63 59 33.4 60

Modifications to the invention as described may be made, as might occurto one with skill in the field of the invention, within the intendedscope of the claims. Therefore, all embodiments contemplated have notbeen shown in complete detail. Other embodiments may be developedwithout departing from the spirit of the invention or from the scope ofthe claims.

1. A polymer-dispersed liquid crystal material for forming apolymer-dispersed liquid crystal optical element comprising: at leastone acrylic acid monomer; at least one type of liquid crystal material;a photoinitiator dye; a co-initiator; and at surfactant.
 2. Thepolymer-dispersed liquid crystal material for forming apolymer-dispersed liquid crystal optical element according to claim 1,further comprising: a polymer matrix formed upon polymerization of thepolymer-dispersed liquid crystal material containing the at least oneacrylic acid monomer in an amount sufficient to increase theconductivity of the polymer matrix and cause a reduction in theswitching voltage.
 3. A polymer-dispersed liquid crystal material forforming a polymer-dispersed liquid crystal optical element comprising: afirst polymerizable monomer and a second polymerizable monomer; at leastone type of liquid crystal material; a photoinitiator dye; aco-initiator; and a reactive di lent; wherein the first polymerizablemonomer and the second polymerizable monomer are polymerized accordingto different chemical processes.
 4. The polymer-dispersed liquid crystalmaterial according to claim 3, wherein the first polymerizable monomeris polymerized using a first polymerizing means and the secondpolymerizable monomer is polymerized using a second polymerizing means.5. The polymer-dispersed liquid crystal material according to claim 4,wherein the first and second polymerizing means are selected from thegroup consisting of: visible radiation, ultraviolet radiation, heat,electron beam, and reactants.
 6. The polymer-dispersed liquid crystalmaterial according to claim 4, wherein at least one of the first andsecond polymerizing means is ultraviolet radiation.
 7. Thepolymer-dispersed liquid crystal material according to claim 3, whereinthe first polymerizable monomer is polymerized at a first time and thesecond polymerizable monomer is polymerized at a second time.
 8. Thepolymer-dispersed liquid crystal material according to claim 3, whereinthe first polymerizable monomer is an acrylate and the secondpolymerizable monomer is a thiol-ene.
 9. The polymer-dispersed liquidcrystal material according to claim 3, wherein the first polymerizablemonomer comprises approximately 30–50% of the polymer-dispersed liquidcrystal material and the second polymerizable monomer comprisesapproximately 5–20% of the polymer-dispersed liquid crystal material.10. The polymer-dispersed liquid crystal material according to claim 3,wherein the first polymerizable monomer is polymerized by a free-radicalchemical reaction anti the second polymerizable monomer is polymerizedby a step-growth chemical reaction.
 11. A switchable optical elementhaving an improved contrast ratio comprising: a polymer matrix having afirst index of refraction; a liquid crystal having a second index ofrefraction; and an alkyl chain additive, wherein the alkyl chainadditive modifies the second index of refraction so that uponapplication of a voltage to the switchable optical element the firstindex of refraction is matched with the second index of refraction andproduces minimum diffraction efficiency within the switchable opticalelement.
 12. A switchable optical element according to claim 11, whereinthe amount of alkyl chain additive within the switchable optical elementis determined based on the following equation:${f_{a:{LC}} = \frac{n_{\min} - n_{p}}{{n_{\min} - n_{a}}\;}},$ wherein,n_(min) is the minimum index of refraction of the liquid crystal, n_(p)is the index of refraction of the polymer matrix, n_(a) is the index ofrefraction of the allyl chain additive, and mixed with the LC will lowerthe effective LC index and f_(a:LC) is the relative-volume-fraction ofalkyl chain additive to liquid crystal.
 13. The switchable opticalelement according to claim 11, wherein the alkyl chain additive isselected from the group consisting of: nonane, decane, dodecane,undecane, tridecane, tetradecane, pentadecane, hexadecane, andheptadecane.
 14. A method for increasing the operating temperature of aswitchable optical element comprising: adding a diluent to apolymer-dispersed liquid crystal material; exposing thepolymer-dispersed liquid crystal material to white light for apredetermined amount of time; and exposing the polymer-dispersed liquidcrystal material to a heat cycle.
 15. The method according to claim 14,wherein the diluent is selected from the group consisting of decanoicacid and dodecanoic acid.
 16. The method according to claim 14, whereinthe heat cycle comprises the following temperatures in order 25 degreesCelsius, 75 degrees Celsius, and 25 degrees Celsius.
 17. Apolymer-dispersed liquid crystal material for forming a switchablepolymer-dispersed liquid crystal optical element comprising: at leastone polymerizable monomer, at least one type of liquid crystal material;a photoinitiator dye; a reactive diluent; and an additive monomeracrylate with branched alkyl chains, wherein the additive monomeracrylate reduces the switching voltage of the polymer-dispersed liquidcrystal optical element.
 18. The polymer-dispersed liquid crystalmaterial according to claim 17, wherein the additive monomer acrylate isselected from the group consisting of t-butyl acrylate, t-butylmethacrylate, isopropyl acrylate, and isopropyl methacrylate.
 19. Apolymer-dispersed liquid crystal material for forming apolymer-dispersed liquid crystal optical element comprising: at leastone acrylic acid monomer; at least one type of liquid crystal material;a photoinitiator dye; a co-initiator; a surfactant; wherein a polymermatrix is formed upon polymerization of the polymer-dispersed liquidcrystal material containing the at least one acrylic acid monomer in anamount sufficient it to increase the conductivity of the polymer matrixand cause a reduction in the switching voltage; and further wherein theat least one acrylic acid monomer is approximately 1.0–1.5 percent ofthe weight of the polymer-dispersed liquid crystal material.
 20. Apolymer-dispersed liquid crystal material for forming apolymer-dispersed liquid crystal optical element comprising: at leastone acrylic acid monomer; at least one type of liquid crystal material;a photoinitiator dye; at co-initiator, a surfactant; wherein a polymermatrix is formed upon polymerization of the polymer-dispersed liquidcrystal material containing the at least one acrylic acid monomer in anamount sufficient to increase the conductivity of the polymer matrix andcause a reduction in the switching voltage; and further wherein theacrylic acid monomer is replaced by at least one of the followingselected from the group consisting of: a reactive ionic material, anaromatic material, a polypyrrole, and an acrylated or methacrylated acidof the type H₂CCH—(CO)O—R—(CO)OH, where R is a linear or branched alkylchain.